SIDR Working Group M. Lepinski
Internet Draft NCF
Intended status: Informational S. Turner
Expires: December 24, 2016 sn3rd
June 22, 2016
An Overview of BGPsecdraft-ietf-sidr-bgpsec-overview-08
Abstract
This document provides an overview of a security extension to the
Border Gateway Protocol (BGP) referred to as BGPsec. BGPsec improves
security for BGP routing.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute working
documents as Internet-Drafts. The list of current Internet-Drafts is
at http://datatracker.ietf.org/drafts/current.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 24, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Lepinski and Turner Expires December 24, 2016 [Page 1]

Internet-Draft BGPsec Overview June 22, 2016
* [I-D.sidr-bgpsec-ops]:
An informational document describing operational considerations.
* [I-D.sidr-bgpsec-pki-profiles]:
A standards track document specifying a profile for X.509
certificates that bind keys used in BGPsec to Autonomous System
numbers, as well as associated Certificate Revocation Lists
(CRLs), and certificate requests.
* [I-D.sidr-bgpsec-algs]
A standards track document specifying suites of signature and
digest algorithms for use in BGPsec.
In addition to this document set, some readers might be interested in
[I-D.sriram-bgpsec-design-choices], an informational document
describing the choices that were made the by the design team prior to
the publication of the -00 version of draft-ietf-sidr-bgpsec-protocol. Discussion of design choices made since the publication of
the -00 can be found in the archives of the SIDR working group
mailing list.
2. Background
The motivation for developing BGPsec is that BGP does not include
mechanisms that allow an Autonomous System (AS) to verify the
legitimacy and authenticity of BGP route advertisements (see for
example, [RFC4272]).
The Resource Public Key Infrastructure (RPKI), described in
[RFC6480], provides a first step towards addressing the validation of
BGP routing data. RPKI resource certificates are issued to the
holders of AS number and IP address resources, providing a binding
between these resources and cryptographic keys that can be used to
verify digital signatures. Additionally, the RPKI architecture
specifies a digitally signed object, a Route Origination
Authorization (ROA), that allows holders of IP address resources to
authorize specific ASes to originate routes (in BGP) to these
resources. Data extracted from a valid ROA can be used by a BGP
speaker to determine whether the origin AS asserted in a received
route has been authorized (by the Internet Number Resource holder) to
originate that route (see [RFC6483] and [RFC7115]).
By instituting a local policy that prefers routes with origins
validated using RPKI data (versus routes to the same prefix that
cannot be so validated) an AS can protect itself from configuration
Lepinski and Turner Expires December 24, 2016 [Page 3]

Internet-Draft BGPsec Overview June 22, 2016
errors by network operators and from certain mis-origination attacks.
However, use of RPKI data alone provides little or no protection
against a sophisticated attacker. Such an attacker could, for
example, conduct a route hijacking attack by appending an authorized
origin AS to an otherwise illegitimate AS path. (See [RFC7132] for a
detailed discussion of the BGPsec threat model.)
BGPsec extends the RPKI by adding an additional type of certificate,
referred to as a BGPsec Router Certificate, that binds an AS number
to a public signature verification key. The corresponding private
key is held by one or more BGP speakers within this AS. Private keys
corresponding to public keys in such certificates are used within
BGPsec to enable a BGP speaker to sign on behalf of its AS. The
certificates thus allow a relying party to verify that a BGPsec
signature was produced by a BGP speaker belonging to a given AS. The
goal of BGPsec is to use such signatures to protect the AS path data
in BGP update messages, so that each BGP speaker can assess the
validity of this data in update messages that it receives.
3. BGPsec Operation
The core of BGPsec is a new optional (non-transitive) attribute,
called BGPsec_Path. This attribute includes both AS Path data as well
as a sequence of digital signatures, one for each AS in the path.
(The use of this new attribute is formally specified in [I-D.sidr-
bgpsec-protocol].) A new signature is added to this sequence each
time an update message leaves an AS. The signature is constructed so
that any tampering with the AS path data or Network Layer
Reachability Information (NLRI) in the BGPsec update message can be
detected by the recipient of the message.
3.1. Negotiation of BGPsec
The use of BGPsec is negotiated using BGP capability advertisements
[RFC5492]. Upon opening a BGP session with a peer, BGP speakers who
support (and wish to use) BGPsec include a newly-defined capability
in the OPEN message [I-D.sidr-bgpsec-protocol].
The use of BGPsec is negotiated separately for each address family.
This means that a BGP speaker could, for example, elect to use BGPsec
for IPv6, but not for IPv4 (or vice versa) routes. Additionally, the
use of BGPsec is negotiated separately in the send and receive
directions. This means that a BGP speaker could, for example,
indicate support for sending BGPsec update messages but require that
messages it receives be traditional (non-BGPsec) update message. (To
see why such a feature is useful, see Section 4.2.)
If the use of BGPsec is negotiated in a BGP session (in a given
Lepinski and Turner Expires December 24, 2016 [Page 4]

Internet-Draft BGPsec Overview June 22, 2016
direction, for a given address family) then both BGPsec update
messages (ones that contain the BGPsec_Path_Signature attribute) and
traditional BGP update messages (that do not contain this attribute)
can be sent within the session.
If a BGPsec-capable BGP speaker finds that its peer does not support
receiving BGPsec update messages, then the BGP speaker must remove
the BGPsec_Path attribute from any update messages it sends to this
peer.
3.2. Update signing and validation
When a BGP speaker originates a BGPsec update message, it creates a
BGPsec_Path attribute containing a single signature. The signature
protects the Network Layer Reachability Information (NLRI), the AS
number of the originating AS, and the AS number of the peer AS to
which the update message is being sent. Note that the NLRI in a
BGPsec update message is restricted to contain only a single prefix.
When a BGP speaker receives a BGPsec update message and wishes to
propagate the route advertisement contained in the update to an
external peer, it adds a new signature to the BGPsec_Path attribute.
This signature protects everything protected by the previous
signature, plus the AS number of the new peer to which the update
message is being sent.
Each BGP speaker also includes a reference, called a Subject Key
Identifier (SKI). The SKI identifies the BGPsec Router Certificate
of the BGP speaker signing the BGPsec_Path attribute. The SKI is
used by a recipient to select the public key (and associated router
certificate data) needed to validate the signature.
As an example, consider the following case in which an advertisement
for 192.0.2/24 is originated by AS 1, which sends the route to AS 2,
which sends it to AS 3, which sends it to AS 4. When AS 4 receives a
BGPsec update message for this route, it will contain the following
data:
* NLRI: 192.0.2/24
* AS path data: 3 2 1
* BGPsec_Path contains 3 signatures :
o Signature from AS 1 protecting
192.0.2/24, AS 1 and AS 2
o Signature from AS 2 protecting
Everything AS 1's signature protected, and AS 3
o Signature from AS 3 protecting
Everything AS 2's signature protected, and AS 4
Lepinski and Turner Expires December 24, 2016 [Page 5]

Internet-Draft BGPsec Overview June 22, 2016
When a BGPsec update message is received by a BGPsec speaker, the
BGPsec speaker can validate the message as follows. For each
signature, the BGP speaker first determines if there is a valid RPKI
Router certificate matching the SKI and containing the appropriate AS
number. (This would typically be done by looking up the SKI in a
cache of data extracted from valid RPKI objects. A cache allows
certificate validation to be handled via an asynchronous process,
which might execute on another device.)
The BGPsec speaker then verifies the signature using the public key
from this BGPsec router certificate. If each of the signatures can be
verified in this fashion, the BGPsec speaker is assured that the
update message it received was propagated via the AS path specified
in the update message.
In the above example, upon receiving the BGPsec update message, a BGP
speaker for AS 4 would do the following. First, it would look at the
SKI for the first signature and see if this corresponds to a valid
BGPsec Router certificate for AS 1. Next, it would verify the first
signature using the key found in this valid certificate. Finally, it
would repeat this process for the second and third signatures,
checking to see that there are valid BGPsec router certificates for
AS 2 and AS 3 (respectively) and that the signatures can be verified
with the keys found in these certificates. Note that the BGPsec
speaker for AS 4 should additionally perform origin validation as per
RFC 6483 [RFC6483]. However, such origin validation is independent of
BGPsec.
The deployment model for BGPsec requires that all ASs in a BGPsec
protected path must be BGPsec speakers. It does not permit BGPsec
protection of an update that propagates through ASs that do not
support BGPsec. In particular, it does not permit what is called
"partial path signing", in which a BGPsec AS attaches a BGPsec_Path
attribute to an unprotected update that was received from a
downstream neighbor.
Partial path signing might be viewed as supplying information about a
portion of a path that could be used in making better routing
decisions, preferring a partially protected route. However, partial
path signing implies that the entire AS path is not rigorously
protected. Rigorous AS path protection is a key requirement of
BGPsec [RFC7353]. Partial path signing also introduces the following
attack vulnerability: If a BGPsec speaker can attach a BGPsec_Path
attribute to an unprotected update, and if BGPsec protected updates
would be preferred to unprotected updates, then a BGPsec speaker can
manufacture any unprotected update it wants and attach a BGPsec_Path
attribute to it, and thereby increase the chance that its
manufactured update will be preferred. Partial path signing then
Lepinski and Turner Expires December 24, 2016 [Page 6]

Internet-Draft BGPsec Overview June 22, 2016
becomes a privilege elevation attack vector, that could be employed
by any BGPsec AS at any point.
The need to avoid introducing that vulnerability forced the stringent
deployment model.
4. Design and Deployment Considerations
In this section we provide a brief overview of several additional
topics that commonly arise in the discussion of BGPsec.
4.1. Disclosure of topology information
A key requirement in the design of BGPsec was that it not disclose
any new information about BGP peering topology. Since many ISPs feel
peering topology data is proprietary, further disclosure of it would
inhibit BGPsec adoption.
In particular, the topology information that can be inferred from
BGPsec update messages is exactly the same as that which can be
inferred from equivalent (non-BGPsec) BGP update messages.
4.2. BGPsec router assumptions
In order to achieve its security goals, BGPsec assumes additional
capabilities in routers. In particular, BGPsec requires adding
digital signatures to BGP update messages, which will significantly
increase the size of these messages. Therefore, an AS that wishes to
receive BGPsec update messages will require additional memory in its
routers to store (e.g., in ADJ RIBs) the data conveyed in these
larger update messages. Additionally, the design of BGPsec assumes
that an AS that elects to receive BGPsec update messages will do some
cryptographic signature verification at its edge router. This
verification may require additional capability in these edge routers.
Additionally, BGPsec requires that all BGPsec speakers support 4-byte
AS Numbers [RFC6793]. This is because the co-existence strategy for
4-byte AS numbers and legacy 2-byte AS speakers that gives special
meaning to AS 23456 is incompatible with the security properties that
BGPsec seeks to provide.
For this initial version of BGPsec, optimizations to minimize the
size of BGPsec updates or the processing required in edge routers
have not been considered. Such optimizations may be considered in the
future.
Note also that the design of BGPsec allows an AS to send BGPsec
update messages (thus obtaining protection for routes it originates)
Lepinski and Turner Expires December 24, 2016 [Page 7]

Internet-Draft BGPsec Overview June 22, 2016
without receiving BGPsec update messages. An AS that sends, but does
not receive, BGPsec update messages, will require much less
capability in its edge routers to deploy BGPsec. In particular, a
router that only sends BGPsec update messages does not need
additional memory to store larger updates and requires only minimal
cryptographic capability (as generating one signature per outgoing
update requires less computation than verifying multiple signatures
on each incoming update message). See [I-D.sidr-bgpsec-ops] for
further discussion related to Edge ASes that do not provide transit.
4.3. BGPsec and consistency of externally visible data
Finally note that, by design, BGPsec prevents parties that propagate
route advertisements from including inconsistent or erroneous
information within the AS-Path (without detection). In particular,
this means that any scenarios in which a BGP speaker constructs such
an inconsistent or erroneous AS Path attribute will break when BGPsec
is used.
For example, when BGPsec is not used, it is possible for a single
autonomous system to have one peering session where it identifies
itself as AS 111 and a second peering session where it identifies
itself as AS 222. In such a case, it might receive route
advertisements from the first peering session (as AS 111) and then
add AS 222 (but not AS 111) to the AS-Path and propagate them within
the second peering session.
Such behavior may very well be innocent and performed with the
consent of the legitimate holder of both AS 111 and 222. However, it
is indistinguishable from the following man-in-the-middle attack
performed by a malicious AS 222. First, the malicious AS 222
impersonates AS 111 in the first peering session (essentially
stealing a route advertisement intended for AS 111). The malicious
AS 222 then inserts itself into the AS path and propagates the update
to its peers.
Therefore, when BGPsec is used, such an autonomous system would
either need to assert a consistent AS number in all external peering
sessions, or else it would need to add both AS 111 and AS 222 to the
AS-Path (along with appropriate signatures) for route advertisements
that it receives from the first peering session and propagates within
the second peering session. See [I-D.sidr-as-migration] for a
detailed discussion of how to reasonably manage AS number migrations
while using BGPsec.
5. Security Considerations
This document provides an overview of BGPsec; it does not define the
Lepinski and Turner Expires December 24, 2016 [Page 8]